I think as we've seen in recent years, the Floridian summer weather is actually a significant disincentive for a lot of vacationers now. Opening in the dead of July would probably have indirect effects on the attendance and perception of the park (i.e grumpy hot people review things less kindly).
Although, if this park is built with a more reasonable amount of shade and cooling that the average Disney park, it shouldn't be as much of an issue.